1. Field of the Invention
The present invention relates to a copyright protection method and an information signal processing system for information signals, such as video signals and audio signals, transmitted via, for example, broadcast media, communication media, or recording media. The invention also relates to an information signal output apparatus, an information signal processing apparatus, an information signal output method, an information signal processing method, and an information signal recording medium used in the above-described copyright protection method and the information signal processing system.
2. Description of the Related Art
Various types of digital content are being widely provided via digital broadcasts, the Internet, or recording media, such as small magneto-optical disks, which are referred to as digital video discs (DVDs) or mini disks (MDs) (trade name)”. Copyright infringement incurred by illegal copying of video signals (image information) and audio signals (audio information), such as music, to be provided as digital content is becoming a problem.
In order to address this problem, information for protecting the copyright may be added to digital content, and by using this additional information, illegal copying may be prevented. To prevent illegal copying, the following cases are considered according to the information sources, such as video signals and audio signals, in which copying may be absolutely prohibited, or copying may be allowed for one generation, but further copying from a copy is prohibited (generation-restricted copying control method).
The first case is applied to original software created and sold by content makers, such as DVD-ROMs. The second case, i.e., the generation-restricted copying control method, is applied to, for example, information to be broadcast in broadcast media and information to be transmitted and received via communication media.
In the generation-restricted copying control method, a technique for effectively controlling copy generations is desirably employed. As copying control methods in which copy generations can be managed, the Copy Generation Management System (CGMS) method and a method using digital watermark processing are proposed for video signals, and the Serial Copy Management System (SCMS) method is proposed for audio signals.
According to the digital watermark processing, information is embedded as noise in parts which are not appreciably perceptible, namely, a redundant portion for music or pictures, of a video signal or an audio signal. The additional information embedded into a video signal or an audio signal according to the digital watermark processing is not easily removed. Moreover, even after performing filtering processing and data compression processing on the video signal or the audio signal, the additional digital-watermark information embedded in the video signal or the audio signal can be detected from image data or music data, respectively.
According to the copying control method using this digital watermark processing, additional information may represent the following four states according to the type of additional information to be embedded, and thus represent the copy generation and the copy-restricted state of the video signal or the audio signal on which the digital watermark information is superimposed.
1. “Copying is allowed (Copy Free)”
2. “copying is allowed for only one generation (Copy Once)”
3. “Copying is prohibited for further generations (No More Copies)”
4. “Copying is absolutely prohibited (Never Copy)”
State 1. “Copying is allowed (Copy Free)”, represents that free copying of video signals and audio signals is allowed. State 2, “Copying is allowed for only one generation (Copy Once)”, indicates that first-generation copying of video signals and audio signals is allowed. State 3, “Copying is prohibited for further generations (No More Copies)”, represents the state of a video signal or an audio signal which was copied from State 2 of the video signal or the audio signal, and indicates that copying is no longer allowed. State 4, “Copying is absolutely prohibited (Never Copy)”, represents that copying is totally prohibited from the original state.
If the digital watermark information superimposed on a video signal or an audio signal corresponds to “copying is allowed for only one generation (Copy Once)”, a digital-watermark-compatible recording apparatus (that is, an apparatus compatible with copying restriction) determines that the video signal or the audio signal can be copied, and performs a recording operation. However, digital watermark information overwritten by the state “copying is prohibited for further generations (No More Copies)” is superimposed on the recorded image data or the recorded audio data. If the digital watermark information superimposed on the video signal or the audio signal to be recorded corresponds to the state “copying is absolutely prohibited (Never Copy)”, the digital-watermark-compatible recording apparatus determines that copying of the video signal or the audio signal is absolutely prohibited, and thus does not perform a recording operation.
In the CGMS method, for an analog image signal, two-bit copy-controlling additional information is superimposed on a specific horizontal interval within a vertical blanking period, and for a digital image signal, two-bit copy-controlling additional information is added to digital image data. Then, the resulting analog image data and the resulting digital image data are transmitted.
The two-bit information in this CGMS method (hereinafter referred to as the “CGMS information”) contains:                [00] . . . copying allowed;        [10] . . . copying allowed for only one generation; and        
[11] . . . copying prohibited (copying is absolutely prohibited).
In the CGMS method, the State “copying is prohibited for further generations” described above does not exist.
When the CGMS information added to a video signal indicates [10], a CGMS-compatible recording apparatus determines that the video signal can be copied, and conducts a recording operation. However, the CGMS information overwritten by [11] is added to the recorded video signal. When the CGMS information added to the video signal to be recorded indicates [11], the CGMS-compatible recording apparatus determines that copying of the video signal is prohibited, and thus does not perform a recording operation.
According to the SCMS method, by using a copy prohibit/allow flag and a category code representing the type of digital audio system, such as a compact disc (CD) or a digital audio tape (DAT), which are transmitted together with a digital audio signal, serial-copying control is performed so that digital copying can be made only once.
That is, in the SCMS method, according to the SCMS information consisting of the copy prohibit/allow flag and the category code to be transmitted together with the digital audio signal, generation-restricted copying control can be performed on the digital audio signal in such a manner that the first-generation digital audio signal can be copied, but the copied digital audio signal cannot be copied any more.
In the above-described CGMS method, the CGMS information is added to a portion different from the information forming a picture (image). Similarly, in the SCMS method, the SCMS information is added to a portion different from the information forming sound, such as music. Accordingly, when using the above-described methods, only a minimal loss is incurred in the picture or the sound, but on the other hand, the picture or the sound is vulnerable to attacks, such as the removal and tampering of the CGMS information or the SCMS information added to a portion different from the information forming the picture or the sound.
In contrast, digital watermark information formed by the digital watermark processing is superimposed in the same time domain and the same frequency band as those of the main information, such as a video signal or an audio signal. The digital watermark information is thus resistant to attacks, such as the removal and tampering of the information. However, at an excessively high superimposition level of the digital watermark information, the image or music (sound) becomes deteriorated. Conversely, at an excessively low superimposition level, it is difficult to detect the digital watermark information, thereby failing to perform speedy and appropriate copying control. If the transfer rate (transmission rate) of the video signal or the audio signal is high, the digital watermark information may not be easily detected.
As discussed above, in superimposing the digital watermark information on a video signal or an audio signal, the problem of tradeoffs is encountered. More specifically, factors, such as the degradation of a picture or music caused by the superimposition of the digital watermark, the data transfer rate, and the reliability of the data, are in a tradeoff relationship, and it is thus difficult to satisfy all the factors.
The CGMS method, the SCMS method, and the digital watermark method using digital watermark information are effective as a copying-restricted control method and a generation-restricted copying control method for video signals and audio signals. However, these methods present the above-described problem, and thus, there is still a demand for a copyright protection method with a higher reliability.